1. Field of the Invention
The present invention relates to an oxygen sensor deterioration diagnosis apparatus used for air-fuel ratio control of an internal combustion engine and for diagnosing the deterioration of an oxygen sensor installed in an exhaust passage of the internal combustion engine.
2. Description of the Related Art
An oxygen sensor installed in an exhaust system of an internal combustion engine and for detecting an oxygen concentration in an exhaust gas has generally such a structure that both an inner and an outer surfaces of a zirconia element are coated with porous platinum electrodes, detects whether the air-fuel ratio is rich or lean as-compared with the theoretical air-fuel ratio on the basis of the oxygen concentration in the exhaust gas, and transmits a signal to an ECU (Electronic Control Unit). The ECU performs feedback control by proportional/integral control or the like using the signal, and performs the control so that the air-fuel ratio of an air-fuel mixture becomes the theoretical air-fuel ratio.
Since the oxygen sensor used in this way is directly exposed to the exhaust gas, combustion dust or the like in the exhaust gas is attached to and covers the porous surface, or the function of the sensor itself is deteriorated by high temperature, and when the deterioration proceeds, the responsibility to the change of the oxygen concentration is deteriorated. In general, in the case where a fuel injection amount and an intake air amount are constant, the output voltage of the oxygen sensor repeats a change at an almost constant inversion period. However, in the case where the deterioration proceeds, the responsibility is deteriorated, so that the inversion period becomes large, it becomes difficult to control the air-fuel ratio to the theoretical air-fuel ratio, and air pollution substances such as hydrocarbon, carbon monoxide, and nitrogen oxide are increased.
Various devices for diagnosing the deterioration of the oxygen sensor as stated above have been proposed, and a technique disclosed in patent document 1 (JP-A-04-072438) is also one of them. In the technique disclosed in this document, a first oxygen sensor is disposed at the upstream side of an exhaust gas purifying three-way catalyst, a second oxygen sensor is disposed at the downstream side thereof, and feedback control is performed while a control point shift of the feedback control by the output of the first oxygen sensor is corrected by the output of the second oxygen sensor, in which in a case where the responsibility of the first oxygen sensor is deteriorated and the inversion period becomes large, the air-fuel ratio control shift is increased, and the absolute level of an increase/decrease level by the second oxygen sensor is increased, and therefore, the absolute level of this correction value is detected and the deterioration of the first oxygen sensor is diagnosed.
Patent document 2 (JP-A-11-166438) discloses a technique in which an upstream side oxygen sensor is disposed at the upstream side of an exhaust gas purifying three-way catalyst, a downstream side oxygen sensor is disposed at the downstream side thereof, an air-fuel ratio correction coefficient is calculated on the basis of the outputs of the respective sensors to perform air-fuel ratio control, an inversion period of the upstream side oxygen sensor is calculated for a specified period of time or a specified number of times, and when this inversion period is longer than a deterioration value, the oxygen sensor is judged to be deteriorated, wherein an intake air amount of an internal combustion engine is integrated for the specified period of time, and a deterioration judgment value is set in accordance with this integrated value, so that even if a load condition of the internal combustion engine is changed, the deterioration judgment can be properly performed.
As stated above, although the inversion period of the oxygen sensor is detected and the deterioration is judged, since the inversion period of the oxygen sensor is intensely dependent on the change of an operation state of the internal combustion engine, such as a fuel injection amount or an intake air amount, there is a problem that in such an operation state as to influence the inversion period of the oxygen sensor, an erroneous diagnosis is performed so that a normal oxygen sensor is judged to be deteriorated. For example, in the case where the fuel injection amount and the intake air amount are periodically increased/decreased, the inversion period of the oxygen sensor is changed in synchronization with the change, and can become such an inversion period that the oxygen sensor is judged to be deteriorated. Thus, the normal oxygen sensor is erroneously diagnosed as being deteriorated.
Besides, in the deterioration diagnosis apparatus in which in the case where the measurement condition of the inversion period is not established continuously for the specified period of time or longer, the inversion period measured up to that time is made ineffective, when the measured inversion period is made ineffective, the normally measured inversion period is also made ineffective, and therefore, there is a problem that the frequency of deterioration diagnosis is decreased.